The present invention relates generally to wireless communications. More particularly, the invention is directed to power control in a code division multiple access (CDMA) wireless communication system.
The relative power used in each data signal transmitted by the base station in spread spectrum communications systems require control in response to information transmitted by each remote unit. The primary reason for providing such control is to accommodate the fact that in certain locations the forward channel link may be unusually disadvantaged. Unless the power being transmitted to the disadvantaged remote unit is increased, the signal quality may become unacceptable. Thus, output power must be controlled to guarantee enough signal strength received at the base station and to maintain good quality audio while minimizing the potential for interference.
Additionally, since a CDMA and new CDMA2000 wideband channel is reused in every cell, self-interference caused by other users of the same call and interference caused by users in other cells represents a limiting factor to the capacity of the system. Moreover, the interference coming from the neighboring base stations may not fade with the signal from the active base station as would be the case for interference coming from the active base station. Due to fading and other channel impairments, maximum capacity is achieved when the signal-to-noise ratio (SNR) for every user is, on the average, at the minimum point needed to support xe2x80x9cacceptablexe2x80x9d channel performance. A remote unit in these situations may require additional signal power from the active base station to achieve adequate performance.
Communication systems are known to employ power control methods which control transmission energy of remote units. Power control in a spread spectrum system serves two main functions. Firstly, because each remote unit""s signal in a spread spectrum system is typically transmitted in the same frequency, a majority of the noise (i.e., inversely proportional to bit energy per noise density, Eb/No, defined as the ratio of signal energy per information-bit to noise power spectral density) associated with a received signal can be attributed to other remote units"" transmissions. The magnitude of noise is directly related to the received signal power of each of the other remote units"" transmissions. Thus, it is beneficial for a remote unit to transmit at a low power level. Secondly, it is desirable to dynamically adjust the power of all remote units in such a way that transmissions are received by the base station with approximately the same power level.
Dynamic power control of the mobile station""s transmitter includes two elements: open loop estimation of transmit power by the mobile station, and closed loop correction of the errors in this estimate by the base station. The closed loop operation is known and will not be discussed here. In open loop power control, each mobile station estimates the total received power on the assigned CDMA frequency channel. Based on this measurement and a correction supplied by the base station, the mobile station""s transmitted power is adjusted to match the estimated path loss, to arrive at the base station at a predetermined level.
CDMA2000 base stations puncture power control bits on the forward fundamental channel (F-FCH) and forward-dedicated control channel (F-DCCH) in an effort to ensure that constant mobile power is received at the base station. Estimates of the signal-to-noise ratio in these power control bits can be used by the mobile to determine whether the base station transmit level matches the targeted Eb/No. The F-FCH can transmit only a continuous stream of traffic data, but transmission of F-DCCH traffic data can be discontinued or resumed on a frame-by-frame basis without notification by the base station. Power control via the continuous F-FCH channel is known and need not be discussed here. However, power control via discontinuous transmission on the F-DCCH makes it more difficult for the mobile station to estimate the appropriate signal power.
What is needed is a reliable implementation for power control using the F-DCCH. In particular, it would be of benefit to provide more reliable performance of forward power control during periods of discontinuous transmission on the F-DCCH. It would also be of benefit if the above improvement could be provided in a simple hardware implementation such as in an application-specific integrated circuit (ASIC).